This invention relates to pipes and more particularly to large diameter plastic pipes of the type used mainly in underground applications.
Plastic pipes for use in underground sewerage and drainage applications are usually manufactured by means of an extrusion process. The wall thickness of these pipes is considerable, in view of the crushing strength required of the pipes. To reduce costs, a pipe of lesser wall thickness has been proposed which is extruded with a continuous, single spiral rib thereon. A continuous spiral rib, however, does not provide as much strength against crushing, as for instance, do discontinuous transverse circumferential ribs. Thus, the very nature of an extrusion process to form a plastic pipe, particularly a large diameter pipe, is unsuited to the making of pipes with discontinuous transverse ribs and accordingly is unsuited to the making of pipes having relatively thin walls yet possessing the required crush resistant characteristics.
Attention was therefore directed to an injection moulding process. Such a process while overcoming the problem outlined above, nevertheless suffers the disadvantage of requiring a separate mould for each different length of pipe. The separate manufacture of pipes of differing lengths would inevitably lead to relatively expensive products, this being particularly the case where pipe lengths of 10 meters or more are required.
The transportation and handling of pipes in the rough and confined spaces generally encountered in drainage installation and the like also requires the pipes to be relatively light weight without impairing their strength characteristics.